Known fractional horsepower motors for hand-held power tools normally have brush holders that guide brushes which are pressed against the commutator by a spring supported on a cover at the end of the holder. The brush holder is positioned within the housing of the motor in a suitable frame. In a known brush holder used especially for home workshop power tools, the casing for the brush is made from a single strip of metal by suitably bending the casing walls so that one casing wall consists of only two lateral strips while its central portion is open. The support cover for the brush spring is formed in the aforementioned case by an upwardly bent tab. The walls of the casing have outwardly bent tongues which serve for connection of an electrical cable to the casing as well as the mechanical attachment thereto.
The casing is placed in its frame in a well-defined form-fitting position so that when the housing of the power tool is assembled, two fingers formed on the housing of the power tool engage the brush casing and fix its position within the casing enclosure. The two fingers of the housing make contact with the two lateral strips of the casing wall, each finger making contact with one of the lateral strips. The electrical current supply cable is provided with a noise suppression choke by soldering.
It is an unfortunate disadvantage of the known brush holder that the carbon brush is frequently jammed in its casing so that the brush spring is unable to press the carbon brush against the armature of the motor. The cause of the jamming may be, for example, incorrect assembly. For example, if the contacting and mounting tabs for the current supply cable are formed improperly, the casing may be deformed, thereby preventing the free motion of the brush. The formation of the support cover for the spring can also cause a deformation of the entire casing. Furthermore, the bending of the current supply tabs out of the surface of the casing wall may produce sharp edges which might intrude slightly into the interior of the casing and cause the carbon brush to jam.
Even when the brush casing and the manner of attaching the current cable are well within tolerances, it is still possible to cause a deformation of the casing by the support fingers of the housing of the tool. If the housing itself is not exactly within tolerances, the support fingers may deform the brush casing during assembly of the housing, thereby also engendering the jamming of the carbon brush.
Even after correct and careful assembly, it has been found that the carbon brush abruptly jams and can no longer be advanced by the spring in the direction of the commutator. The cause of the jamming is that the brush casing carries the electrical current and may be heated to high temperatures. During such heating, the two support fingers which are integral with the housing of the tool and are normally constructed of plastic are sintered slightly in the vicinity of the point of attachment to the sides of the brush casing. The very slight sintering suffices to cause the central part of the support finger to protrude through the open part of the casing toward the carbon brush. Thus the brush is forced against the aforementioned sharp edges on the tabs of the current supply cable, causing the brush to jam.